chapter 3 experimental - fu-berlin.de

Chapter 3: Experimental

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Chapter 3 Experimental

3.1 The UHV Chamber and the LT-STM

The Ultra High Vacuum (UHV) chamber and the LT-STM used in this work have

been built by K. Schaeffer, S. Zöphel, and G. Meyer.75-78 The UHV chamber is

shown in Fig. 3.1. It consists of two parts separated by a valve: The preparation

chamber and the measuring chamber, containing the LT-STM.

The preparation chamber is equipped with an ionisation gauge and a quadrupol

mass spectrometer for rest gas analysis, an ion gun and a gas inlet system (Ne) for

sputtering the sample, a Low Energy Electron Diffraction (LEED) system for

characterization of the sample, and an evaporator (Kentax TCE-BS) for molecular

beam epitaxy (MBE) of organic molecules. The evaporator consists of four quartz

glass crucibles each one heated by a surrounding tungsten wire and equipped with

Ni/Cr-Ni-thermocouples. A shutter closes all cells beside the one that is in use.

The evaporator can be retracted and transferred to air for changing the substances

and transferred back to the chamber, without breaking the vacuum in the UHV

chamber. A load lock is attached to the preparation chamber for changing samples

and a storage system inside the preparation chamber can store up to two samples.

The turbo-molecular pump of the load lock is also used for pumping the UHV

chamber during sputtering or bake out. Otherwise the UHV chamber is pumped


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by an ion getter pump and a titanium sublimation pump (TSP); the base pressure

is about 1×10-10 mbar.

Fig. 3.1. The UHV chamber. The preparation chamber is to the left and the STM-chamber is to the right.78 The MBE cell and the load lock are on the opposite side at the preparation chamber.

The samples are mounted on sample holders, attached to a button heater and a

Ni/Cr-Ni-thermocouple (see Fig. 3.2). The sample holder can be transferred to the

interlock, the sample storage, and the STM by means of a manipulator. The

manipulator allows movement in all directions and rotation around its length axis

and is also used to bring the sample into the different positions needed for

preparation and characterization. Besides heating by means of the button heater,

the sample can be cooled on the manipulator to approximately 15 K using liquid

helium.


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The measuring chamber contains another ionization gauge and the LT-STM. The

LT-STM is cooled by a bath cryostat consisting of a liquid nitrogen tank (13.3

litre) and a liquid helium tank (4.3 litre). In the usual working mode the sample is

maintained at a temperature of (8±1) K and the nitrogen and helium tanks have to

be refilled every 44 hours. The STM is thermal weakly coupled to the cryostat

and can be heated up to room temperature using a Zener-diode. For mechanical

isolation the STM is hanging on stainless steel springs and is damped by an eddy

current break. For further damping, especially against low frequency vibrations,

the whole chamber sits on four pneumatically damped feet.

Sampleholder

Sample

Heater

Fig. 3.2. Drawing of the STM inside the radiation shields.78 The piezoelectric tubes and contact pads indicated. The sample holder can be seen in the front.

The STM is a modified Besocke type79, 80 (Fig. 3.2). Three outer tube piezos and

one central tube piezo are used to move the tip. The tip sits at the end of the

central piezo, which is mounted on a plate with three ramps. The ramps rest on

sapphire balls on the three outer piezos. By employing the three outer piezos und

using slip-stick-motion, the tip is moved on the mm scale as needed for the coarse

approach. When the tip is in tunnelling contact with the sample, the central piezo

is employed to regulate the tip height (z-direction). The scanning movement


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parallel to the sample (x- and y-direction) can be done either by the central piezo

(main scanning mode) or by the outer piezos (coarse scanning mode). The latter

mode was usually used in this work. The tunnelling current is measured at the tip

side while the bias voltage is applied to the sample, i.e. the bias always refers to

the sample with respect to the tip.

The STM is controlled by a PC using a homemade software program

(PSTMAFM). The computer is connected to the analog electronics by a DSP

(digital-signal-processor) and a D/A-A/D converter (digital-to-analog and analog-

to-digital). The electronics consists of a high voltage amplifier for the movement

of the piezos, a buffer amplifier for the tunnelling bias voltage, an amplifier for

the tunnelling current and a lock-in amplifier, used for STS (scanning tunnelling

spectroscopy). The D/A converters are used to control the piezos and the tunnel-

ling voltage, while the A/D converters are used to read out the amplified tunnel-

ling current and the lock-in signal.

All STM images shown in this work have been measured at temperatures between

7.3 K and 9 K and the piezo constants have been calibrated temperature depend-

ent using the lattice parameters of the Cu(111) and the Cu(211) surface in

atomically resolved images. For a more detailed description of the LT-STM see

Ref. 78.


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3.2 The Substrates

3.2.1 Cu(111)

The Cu(111) surface is the hexagonal close packed face of a Cu single crystal,

which has a face centred cubic (fcc) structure with a lattice constant of

Åa 61.3= .81 The work function of Cu(111) is 4.94 eV.82 The nearest neighbour

distance is Åa 55.22/ = and the single step height is Åa 08.23/ = . Later in

this work step dislocations will be produced, therefore it is important to know that

gliding preferentially occurs along the <110> slip systems leading to dislocation

steps of height Ån 08.2× on the surface. There are two unequal types of such

steps, i.e. A-type steps, exhibiting 100 facets and B-type steps, showing 111

facets, respectively. Once the exact orientation of the crystal is known, these steps

can be distinguished by their orientation. The orientation of the crystal can be

determined by means of STM, by observing the orientation of steps of 1/3 and 2/3

of the usual step height.83 In Fig. 3.3 an atomically resolved STM image of the

Cu(111) surface is shown, as used for calibration of the piezos in this work.

[011]

5 Å2.55Å

Fig. 3.3. STM image of the Cu(111) surface with atomic resolution. Tunnelling parameters are I = 4.4×10-7 A, U = -50 mV, T = 7.5 K. A hexagonal unit cell is indicated.


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As all noble metal (111) surfaces, Cu(111) exhibits a Shockley surface state.84

Surface states exist on the close-packed faces of noble metals, because of a band

gap along the Γ-L direction of their bulk band structure.84, 85 Due to the presence

of the crystal surface, bulk forbidden electronic single-particle states arise, leading

to a band in the corresponding projected bulk band gap. Thus a 2D nearly-free

electron gas is formed by the Shockley state, which can be directly imaged as

standing wave patterns by STM, as first observed by Eigler et al.86, 87 (see Fig.

2.3). The otherwise half filled surface state is populated/depopulated by electrons

tunnelling from/to the tunnelling tip. These electrons are scattered at defects as

adsorbates or step edges causing interference patterns. Heller et al. proposed a

multiple scattering method to simulate the observed standing wave patterns88,

which will be also applied in this work (see section 5.3). The dispersion relation

of these surface state electrons could be determined with high accuracy by STS

measurements and was found to be almost free electron like.89 The band edge

energy is EΓ = -420 eV (with respect to the Fermi level) and the effective mass is

m* = 0.40 me. Significant deviations from parabolic dispersion appear at energies

higher than 2 eV above the Fermi level.89 Moreover, lifetime90 of surface state

electrons, as well as scattering phase shifts and absorption coefficients for various

systems have be determined.

3.2.2 Cu(211)

A model of the Cu(211) surface is shown in Fig. 3.4. The surface ideally consists

of (111) facets separated by (100) steps running into the ]101[ direction. The

intrinsic steps, showing a distance of a2 = 6.26 Å, are usually resolved in STM

measurements, while atomic resolution along the close-packed step edges

(a1 = 2.55 Å), as in Fig. 3.5 is only obtained by a modified tip.


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Fig. 3.4. Sphere model of the Cu(211) surface with a (111) and a (311) terrace step. The crystalline faces of the micro facets are indicated.

]111[

]101[

10Å

a2

a1

Fig. 3.5. STM image of the Cu(211) surface with atomic resolution. Tunnelling parameters are I = 3.8 nA, V = 86 mV, T = 7.3 K. In the centre of the image a single adatom is visible. The surface lattice parameters a1 = 2.55 Å and a2 = 6.26 Å are indicated.

The step separation is 6.26 Å and the miscut angle towards the (111) plane is

19.5°. Terrace step edges run preferably in ]101[ direction, that is parallel to the

intrinsic steps. The height of a single step is 0.74 Å. It can be distinguished

between two different types of ]101[ directed terrace steps. As illustrated in the

model in Fig. 3.4, the (111) steps lead upwards while the (311) steps lead

downwards with respect to the ]111[ direction.


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3.3 The Molecules

The molecules that are investigated in this work have all been synthesised by A.

Gourdon and collaborators at the Nanoscience Group, CEMES-CNRS, Toulouse,

France. The molecules are specially designed to study the possibilities of employ-

ing single molecules as electronic devices or as nano-machines. From the initial

stage on, there have been intense collaborations between chemists and theoreti-

cians in Toulouse and experimentalists in our group to develop these molecules.

3.3.1 Lander Type Molecules

Lander type molecules were specially designed to study single molecular wires on

metallic surfaces by STM.91 The Lander, also called Single Lander (SL, C90H98)

consists of a planar polyaromatic molecular board, that constitutes the wire part,

and four spacer groups, which are attached to the wire in order to decouple it from

the metallic surface underneath (the molecule has been named due to similarities

with landing interplanetary space crafts).

The molecular board is a polyaromatic planar system with overlapping π-orbitals.

A HOMO-LUMO gap of Eg = 1.24 eV has been calculated for an

Oligo(cyclopenta-naphto-flouroathene) molecular wire, as the Lander molecular

board, by Magoga and Joachim.92 At low bias voltage (inside Eg ) they predicted

the conductance G of a metal-molecule-metal junction to follow an exponential

law

( )LGG γ−= exp0 , (3-1)

where L is the inter-electrode separation, γ is the inverse damping length,

depending only on the intrinsic electronic properties of the molecular wire, and

the prefactor G0 is depending on the adsorption geometry of the molecule at the

metal electrodes. In the case of an Oligo(cyclopenta-naphto-flouroathene)

molecular wire, the two important parameters have been calculated by means of

ESQC to be γ = 0.207 Å-1 and G0 = 1.27 × 10-7 Ω-1. 92 Although these values show


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a rather large resistance of the molecular wire, the use of oligomers of well

defined length is still promising because it permits the synthesis of nano-devices

by including the chemical functionality in the wire.92

The four spacer groups (legs), attached to the board by insulating σ-bonds, are

3,5-di-tert-butyl-phenyl (TBP) groups. Fig. 3.6 shows the Lander molecule, which

is approximately 17 Å long (in the direction of the molecular board) and 15 Å

wide including van-der-Waals radii. The design ensures the board to be elevated

from a flat surface, therefore reducing the interaction of the wire part with the

surface and allowing lateral manipulation with the STM tip. Moreover, the

molecule has a rigid aromatic platform (naphthalene end group) located beyond

the spacers, which can be employed to study the electric contact of the wire with a

nanostructure or step edge on a metal surface.

polyaromatic board

3.5-di-tert-buthylphenyl groups

naphthalene end group (b)(a)

Fig. 3.6. Chemical structure of a Single Lander (a), with the important functional-ized parts indicated. Sphere model (b) of the molecule in the gas phase conforma-tion.

First STM experiments with Lander molecules have been performed on Cu(100)

at room temperature in the group of Gimzewski at IBM Zürich.16 Recently, the

adsorption of Lander molecules on Cu(100) has been described in detail by

Kuntze et al.93 Many observations are characteristic for Lander molecules in

general and remain valid for other Cu substrates: Cu(110)94-96, Cu(111)97 and

Cu(211)98. These general results are reviewed in the following.


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Θ

(a)

(c)

Φ

(b)

Fig. 3.7. Sphere models of a Lander molecule: (a) top view, (b) front view, (c) side view. The angles describing the deformation of the σ-bonds are indicated. Rotation out of the molecular plane is labelled Θ, bending of the σ-bonds is described by Φ.

(a) (b)

Φ

Fig. 3.8. Lander molecules adsorbed on a Cu(100) terrace. (a) STM image, I = 0.1 nA, U = 1.0 V, T = 4.6 K. Image size (9 nm)². Molecules in (A) parallel legs and (B) crossed legs conformation. (b) Ball model, the molecule is viewed from the front. (c) Illustration of the schematic conformations. The molecule is symbolized as a board with four legs (TBP-groups) attached. From ref.93


33

Lander molecules are imaged as four bright lobes corresponding to the four TBP

side groups of the molecule, while the molecular board contributes little to the

tunnelling current. Therefore the molecular board is normally not visible in STM

images, as shown in Fig. 3.8(a).

On defect free terraces, Lander molecules are found in two different conforma-

tions, called crossed legs and parallel legs. Due to the molecule-substrate interac-

tion, the conformation of the molecule is altered with respect to its gas phase

conformation. The board to surface distance decreases from 5 Å, which would be

expected for the gas phase conformation, to 3.7 Å for the adsorbed molecule. The

main deformation is due to tilting and bending of the σ-bonds between molecular

legs and board. The bending of the σ-bonds, allowing a decreased board-substrate

distance, can be seen in the model in Fig. 3.8(b). The rotation of the legs can

occur in different directions for the pairs of legs on each side of the board, as is

indicated in models in Fig. 3.8(c), giving rise to the different conformations. The

two legs on one side of the board are always rotated in the same direction owing

to steric hindrance. The parallel legs conformation corresponds to a rotation of the

legs on both sides of the board in the same direction and the molecule is imaged

in an overall square shape. If the legs on both sides of the board rotate in opposite

directions the molecule is adsorbed in the so called crossed legs conformation and

is imaged in an rhombic shape. A third conformation is a mirror symmetric

enantiomer of the latter. Although the molecule is not chiral in the gas phase,

chirality is induced by adsorption and the anti-symmetrical rotation of legs.

(a) (b)

Fig. 3.9. Lander molecules adsorbed at a monoatomic step. (a) STM image: I = 0.1 nA, U = 1.0 V, T = 4.6 K. Image size (6.7 nm)². (b) Ball model, the mole-cule is viewed from the front. The molecular board is parallel to the step.93


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The adsorption of Lander molecules at monoatomic step edges of the Cu(100)

surface occurs with the molecular board parallel to the step, with two legs located

on the lower and two legs on the upper terrace, as shown in Fig. 3.9.93

On Cu(110) a surface induced restructuring was observed by Besenbacher and

collaborators95, 96, consisting of double rows of Cu adatoms under the board of

Lander molecules. The restructuring was proven by laterally manipulating Lander

molecules away from their adsorption sites, as shown in Fig. 3.10. The nanostruc-

ture formed under the Lander molecule is (8±1) atoms long and two atoms wide.

Fig. 3.10. (A to D) Manipulation sequence of the Lander molecules from a step edge on Cu(110). The arrows show which molecule is being pushed aside; the circles mark the tooth-like structures that are visible on the step where the mole-cule was docked. Scan parameters: I = 0.47 nA, U = 1.77 V, T = 100 K. Image size (13 nm)². Manipulation parameters: I = 1.05 nA, U = 55 mV. (E) STM image showing the characteristic two-row width of the tooth-like structure after removal of a single Lander molecule from the step edge (I = 0.75 nA, U = 1.77 V, image size 2.5 × 5.5 nm²). From ref.96

Besides the described SL (Fig. 3.11(a)) other Lander derivatives have been

synthesized. Two of them have been investigated in the present work; they are

called Reactive Lander (RL, C94H98, Fig. 3.11(b)) and Violet Lander (VL,

C108H104, Fig. 3.11(c)). After synthesis the molecules are in the form of a orange,

red, and violet crystalline powder in case of the SL, RL, and VL, respectively

(therefore the naming of the VL). All these Lander type molecules exhibit four


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identical lateral di-tert-butyl-phenyl “leg” groups (TBP) which hold the wire

parallel to the metal surface. The molecular wire “board” is an aromatic planar

conjugated π-system in all cases, but has different lengths for the different Lander

type molecules: 17 Å for the SL, 20 Å for the RL, and 25 Å for the VL, respec-

tively. The width of all molecules is approximately 15 Å.

(a) SL

15 Å

17 Å

15 Å

15 Å

20 Å

25 Å

(b) RL(c) VL

Fig. 3.11. Structure models and van der Waals dimensions of the different types of Lander molecules. Single Lander (SL), Reactive Lander (RL), and Violet Lander (VL). The aromatic board of the RL is extended at the ends with respect to the SL, while the board of the VL is extended in the middle part. The four di-tert-butyl-phenyl side groups (legs) are identical for all three molecules.

The idea behind the synthesis of the RL molecule was to add reactive sites to the

ends of the molecular board in order to connect single molecules to an extended

molecular wire by means of STM induced chemical reactions. The coupling

reaction would be a 2+2 cyclo-addition. However, we found that the geometric

conformation which is required for the reaction (the end groups above each other)

could not be obtained when the molecules are adsorbed.

In case of the VL molecule99, the wire part of a molecule is extended and the

spacing between the legs is increased, reducing the steric hindrance between the

molecular legs. Adsorption of the VL molecule was studied on Cu(100)99,

Cu(110)100, and Cu(111)101. In the case of VL/Cu(110) it has been demonstrated

by Otero et al. that the diffusion of VL molecules can be switched on and off by

STM induced molecular orientations.102


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3.3.2 HBC, HPB, and Derivatives

In the second part of this work, self-ordered processes of organic molecules are

investigated. This is a technological important issue, since self-assembled organic

thin films have numerous applications, e.g. in heterogeneous catalysis103, sen-

sors104, and as interfaces in medical implants105. In terms of the economic

fabrication of nano-structured surfaces and ordered thin films needed in molecular

electronic and optic devices104, self-assembly appears to be a practical strategy

since it allows fabrication in a parallel geometry. Hence, for the design of

molecules for applications in organic film technology, knowledge on the correla-

tion between the chemical molecular structure and the monolayer epitaxial growth

is of high interest.

(a) HPB

(d) HB-HPB

19 Å

17 Å

(b) HBC

12.5

Å

14.5 Å

m

m

(c) HB-HBC

19 Å

17 Å

m

14.5 Å12

.5 Å

m

Fig. 3.12. Structure models and van der Waals dimensions of the investigated molecules. The dashed arrows indicate the molecular axis mv . (a) HPB, due to steric interaction the outer phenyl rings are tilted around the sigma bonds to the central benzene ring; (b) HBC, an entirely planar aromatic system; (c) HB-HBC and (d) HB-HPB are both equipped with six additional tert-butyl groups at the outer aromatic rings and have the same internal structure as (b) and (a) respec-tively.


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The aim here is to study the effect of specific chemical groups on adsorption,

growth, and self-ordering of large organic molecules. In this work four slightly

different molecular derivates are chosen. This molecules are hexa-peri-

hexabenzocoronene derivatives, which have been proved interesting for organic

film photovoltaic technology106 and molecular electronics107-111.

The molecules are all showing a six-fold symmetry, therefore matching the

Cu(111) substrate symmetry. Also the distances within the molecule, i.e.

(2.5±0.1) Å between adjacent benzene rings, fit the lattice constant of Cu(111):

2.55 Å. The molecules, which are investigated here are: Hexa-peri-

hexabenzocoronene (HBC, C42H18), hexa-tert-butyl-hexabenzocoronene (HB-

HBC, C66H66), hexaphenylbenzene (HPB, C42H30), and hexa-tert-butyl-

hexaphenylbenzene (HB-HPB, C66H78). The corresponding structure models are

shown in Fig. 3.12.

The HBC molecule is well known in literature32, 112-115, while the other molecules

have been specially synthesised for this work by A. Gourdon. Highly ordered

layer by layer growth of HBC has been shown upon deposition onto pyrolytic

graphite (0001) and on molybdenum disulfide, MoS2(0001).116 This growth mode

continues up to a layer thickness of at least 10 nm.114 HBC has been studied in

monolayer and multilayer coverage also on various noble metal surfaces, e.g.

Au(111)32, 113, 115, Au(100)117, and Cu(111)32. Generally, on clean metals (under

UHV conditions) the formation of highly ordered monolayers with the HBC

molecular planes oriented parallel to the substrate has been reported. Higher

coverage leads to columnar stacking of HBC molecules with highly oriented

HBC-adlayers up to a thickness of 2 nm on Cu(111) and Au(111).32

The Hexa-peri-hexabenzocoronene (HBC) is a large molecular segment of a

graphite plane, which can be deposited using sublimation. The molecule is large

enough to add functional molecular side groups, making these class of molecules

very interesting for applications. Self-organization of hexaalkyl-substituted

derivates of HBC into a columnar mesophase in organic solvents has been

demonstrated, leading to one-dimensional conductors with a very high charge

carrier mobility, i.e. molecular nanowires108. Recently, Jäckel et al. reported on a


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prototypical single-molecule chemical-field-effect transistor, using an HBC

derivative in the tunnel junction of an STM.111 Moreover, discrete tubular self-

assembled nano-tubes have been grown from HBC derivatives.109

The hexaphenylbenzene (HPB) molecule is a pre-stage in the synthesis of the

HBC molecule. In case of the HPB, the bonds between the outer phenyl rings are

not established in comparison to HBC. Hydrogen atoms are bond to the corre-

sponding carbon atoms inducing steric hindrance between adjacent phenyl rings.

In vacuum, all phenyl groups are rotated around their σ-bonds by approximately

45° with respect to the central benzene ring, leading to a propeller shape form of

the molecule. It has been shown that HPB can be transferred into HBC by thermic

cyclodehydrogenation on a Cu(111) surface.118

In case of the larger HB-HBC and HB-HPB molecules, tert-butyl side groups are

added to the previously described HBC and HPB molecules. These side groups

are similar to the di-tert-butyl side groups, that are used in the case of the Lander

molecules. The adding of such side groups is expected to alter the intermolecular

forces of adsorbed molecules and therefore to influence molecular ordering. Large

differences in the adsorption and monolayer structure have been observed due to

the adding of tert-butyl side groups in the case of decacyclene (DC) and hexa-

tert-butyl decacyclene (HB-DC) on Cu(110) as has been shown by Schunack and

collaborators.27, 28


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3.4 Sample Preparation

The sample substrates are prepared by multiple cycles of sputtering with 1.3 keV

Ne ions at room temperature for 20 minutes followed by annealing to 770 K for

five minutes. The molecules are evaporated from a home built Knudsen cell and

from commercial Knudsen cells (Kentax TCE-BS). The evaporation temperatures

of the molecules are shown in Table 3.1.

Molecule Chemical formula Sublimation temperature

(±20 K)

SL C90H98 620 K

RL C94H98 650 K

VL C108H104 660 K

HBC C42H18 620 K

HB-HBC C66H66 570 K

HPB C42H30 420 K

HB-HPB C66H78 520 K

Table 3.1. Chemical formulas and used sublimation temperatures for the investi-gated molecules. The sublimation temperatures correspond to an evaporation rate of 10-4 ML/s.

The preparation is monitored via a quartz crystal microbalance, that has been

calibrated using the STM. The sensibility of the microbalance is in the order of

5 × 10-4 monolayer (ML) and the error in the coverage is about 30% as checked

by STM. In this work 1 ML is defined as the amount of molecules, that would

cover the sample completely assuming the densest observed monolayer structure.

In all cases the preparations are in the submonolayer regime, a typically adsorbed

dosage is 0.01 ML, the typical evaporation time is 10 min. For investigations of

monolayer formation (i.e. in the case of HBC derivates) sometimes a higher

coverage, up to 0.3 ML is chosen, while for the investigation of standing wave

patterns and manipulation experiments with single molecules a lower coverage, in


40

the order of 1 × 10-3 ML is used. In these cases the evaporation time is kept at

values between 5 and 15 minutes, but the evaporation temperature is varied, using

the calibrated microbalance to achieve the desired flux (i.e. evaporation rates are

in the orders of 10-6 ML/s to 10-3 ML/s). At the sublimation temperature the flux

increases by about one order of magnitude with a temperature increase of 10 K. It

is important to mention that the size of the molecules that can be evaporated by

molecular beam deposition from a hot crucible is limited. In general, the sublima-

tion temperature of molecules, which are only loosely bound by van der Waals

forces, increases with the molecular weight. As shown in Table 3.1 the HBC

molecule is an exception to this general rule since HBC molecules grow in layers

like graphite thus increasing the sublimation temperature due to the rather strong

van der Waals forces between the molecular planes. If the temperature needed for

sublimation of the molecule is higher than the temperature at which the inner

molecular bonds dissociate, molecules are evaporated in fragments. This critical

temperature seems to be nearly reached in the case of the Violet Lander (VL), the

largest molecule investigated in this work. A crucible temperature of at least

660 K was needed to evaporate the VL molecules and STM measurements show

that a large amount of fragments were sublimed together with intact molecules.

Another important preparation parameter is the sample temperature. When the

sample temperature is low enough, molecular diffusions is frozen in and mole-

cules are pinned at their initial adsorption places. For dosages in the far sub-

monolayer regime, this will result in a random distribution of single molecules on

the surface, useful for the investigation of single molecules and their properties as

surface state scatterers. However, preparation at low sample temperature bears the

disadvantage of increasing the sticking coefficient also for unwanted rest gas

molecules, e.g. CO, which might contaminate the sample. On the other hand no

contamination is observed inside the LT-SM within weeks, because of the cold

shields surrounding the sample. Contamination is thus a problem only as long as

the sample is inside the preparation chamber, especially when the sample

temperature is low and the ambient pressure is increased due to the heating of the

Knudsen cell. At higher sample temperatures, molecules can diffuse until they

reach preferred adsorption sites. This can result in selective adsorption at certain


41

substrate sites, molecular self-organisation, or, involving also the diffusion of

substrate atoms, molecular induced restructuring.

Since the manipulator allows cooling and heating the sample, the molecular

adsorption could be investigated temperature dependent. Therefore, sample

temperatures were chosen between 70 K, freezing in the diffusion of SL mole-

cules, and 330 K, allowing surface diffusion of all investigated molecules. The

ambient pressure during evaporation is in the order of 10-9 mbar. After prepara-

tion the sample is cooled down to 20 K on the manipulator and transferred to the

STM. All shown STM images have been measured at temperatures between 7 K

and 9 K.


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chapter 3 experimental - fu-berlin.de

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